Waste waters containing organic matters have been so far treated mostly by the activated mud method. However, polyvinyl alcohol (designated hereinafter as P.V.A.), if present in the water, is hardly decomposed by the method. It is better for such waters to be treated with boric acid or borax in the presence of an inorganic salt, for example, sodium sulfate, to coagulate and separate P.V.A. in the form of a gelatinous compound with boric acid. In this case, however, the mother liquor from which P.V.A. has been recovered contains a small amount of borate as well as inorganic salts. These salts should not be discarded, even if the amount lies within the permitted range of pollution control, from the point of recycling natural resources.